Chapter 27: Development and Inheritance

Development begins when a single sperm penetrates the oocyte after acrosomal enzymes from multiple sperm break down protective layers, creating a zygote containing the full diploid chromosome complement. The zygote then undergoes cleavage divisions to form a morula, followed by blastocyst formation with distinct inner cell mass tissues. Around the second week, gastrulation redistributes cells into three germ layers, each destined to form specific organ systems: the ectoderm generates nervous tissue and outer skin, the mesoderm produces muscles and circulatory structures, and the endoderm lines respiratory and digestive tracts. Prenatal development spans three trimesters, with the first representing the most vulnerable period when major structures form, the second allowing organ system maturation, and the third characterized by substantial fetal weight accumulation and functional preparation for independent life. Throughout pregnancy, the placenta functions simultaneously as a nutrient and waste exchanger and as an endocrine structure secreting hormones that maintain pregnancy and modify maternal anatomy. Pregnancy induces significant maternal changes including expanded blood volume and elevated metabolic rates that persist throughout gestation. Labor progresses through three distinct stages beginning with cervical dilation, proceeding through forceful contractions that deliver the fetus, and concluding with placental expulsion. After birth, development continues through defined postnatal stages marked by rapid growth and maturation, with puberty representing a critical transition driven by hormonal signals from the hypothalamus and pituitary that initiate gamete production and visible secondary characteristics. The chapter concludes by distinguishing genotype from phenotype and explaining how traits transmit through simple Mendelian patterns, polygenic mechanisms involving multiple loci, sex-linked inheritance associated with X chromosome location, and chromosomal abnormalities resulting from nondisjunction. Additionally, epigenetic mechanisms that modify gene expression without altering DNA sequence itself represent an important dimension of inheritance not visible in standard genetic crosses.